1. Field of the Invention
The present invention relates to a belt for an image forming apparatus, which is mounted in an image forming apparatus, such as a photocopying machine, and a printer, and relates to an image forming apparatus using the belt.
2. Description of the Related Art
Conventionally, a seamless belt has been used as a member of an electrophotographic device for various uses. Especially, recent full-color electrophotographic devices employ an intermediate transfer belt system, in which developed images of four colors, yellow, magenta, cyan, and black, are superimposed on an intermediate transfer member temporarily, followed by being collectively transferred onto a transfer medium, such as paper.
As for the aforementioned intermediate transfer belt, a system using developing units of four respective colors to one photoconductor has been used, but this system has a problem that a printing speed thereof is slow. Accordingly, to achieve high speed printing, a quadruple-tandem system has been used, where the tandem system includes providing photoconductors of four respective colors, and an image of each color is continuously transferred to paper. In this system, however, it is very difficult to accurately position images of colors to be superimposed, because the position of paper changes as affected by the environment, which causing displacement of the colors in the image. Accordingly, currently, an intermediate transfer belt system has been mainly adapted for the quadruple-tandem system.
Under the circumstances as mentioned above, the higher requirements for properties (high speed transferring, and accuracy for positioning) of an intermediate transfer belt have been demanded than before, and therefore it is necessary for an intermediate transfer belt to satisfy these requirements. Especially for the accuracy for positioning, it has been required to inhibit variations caused by deformation of an intermediate transfer belt itself, such as stretching, after continuous use thereof. Moreover, an intermediate transfer belt is desired to have flame resistance as it is provided over a wide region of a device, and high voltage is applied thereto for transferring. To satisfy these demands, a polyimide resin, polyamide imide resin, or the like, that is a highly elastic and highly heat resistant resin, has been mainly used as a material of an intermediate transfer belt (base layer).
An intermediate transfer belt formed of a polyimide resin is however has high surface hardness, as the resin constituting the belt has high hardness. When a toner image is transferred, therefore, high pressure is applied to a toner layer. As a result, the toner is partially aggregated, and formation of so-called a partially-missing image, where part of an image is not transferred, may be caused. Moreover, the intermediate transfer belt having high hardness has inferior correspondence to a member to be in contact in a transferring section, such as with a photoconductor or a sheet. Accordingly, a contact failed area (void) may be partially caused in the transferring section, causing transfer unevenness.
In addition, there have been recently more occasions that images are formed on various types of a sheet in a full-color electrophotographic system. Not only a general smooth sheet, various sheets from a sheet of high smoothness with slippery surface, such as coated paper to a sheet having surface roughness, such as recycle paper, embossed paper, Japanese paper, and Kraft paper, have been often used. It is important that an intermediate transfer belt has correspondence to sheets of these different surface configurations. If the intermediate transfer belt has insufficient correspondence, unevenness in density and color tone corresponded to the convexo-concave surface configurations of the sheet appears in the resulting image. To solve this problem, various intermediate transfer belts have been proposed to laminate a relatively flexible rubber elastic layer onto a base layer. Such rubber elastic layer however has greater thermal shrinkage than the base layer, which causes a problem that the belt tends to curve outward (to the side of the elastic layer).
Hitherto proposed was to coat an elastic layer provided on a transfer roller, not an intermediate transfer belt, with beads having diameters of 3 μm or smaller (see Japanese Patent Application Laid-Open (JP-A) No. 09-230717). When this structure is applied to a belt, however, the resulting belt has poor durability, and causes detachment of the particles, and running failures due to the edge curving of the belt. Therefore, such modification of the belt is not sufficient to solve the aforementioned problem.
Moreover, proposed has been to fix edge portions of an intermediate transfer belt to reduce edge curving (see JP-A No. 2000-293045). This technique is, however to press the curved edge externally by bonding with a tensile strength tape, rather than preventing occurrence of edge curling itself. A belt of this structure does not have sufficient durability required for use in current electrophotographic devices.
Further, proposed has been to increase a thickness of an edge portion of an intermediate transfer belt compared to a thickness of a center portion thereof (see JP-A No. 200783424). In the case where a thickness difference is made in a relatively hard material, such as a base material formed of a polyimide resin, transfer unevenness tends to occur, or an edge of the belt tends to be torn.
The present inventors have disclosed a cylindrical intermediate belt transfer member containing a belt base, a binder layer, and a particle layer (see Japanese Patent (JP-B) No. 4430892). However, this technique is not necessarily for disclosing a certain technical solution for preventing both edge curling of the belt with respect to the width to direction of the belt.